Days of Future Past could just as well be called 'Bryan Singer reclaims the X-Men'.
For a start, he brings back the theme tune from X2. That's just a hint that this may be the seventh movie, but it's really his X-Men 3. Secondly - and this surprised me - Singer shows the Weapon X footage from X2, not X-Men: Origins. It's a big middle finger to that movie - which he then wipes from continuity completely, and sets it up so he can do Weapon X again himself if he wants.
And then Scott and Jean are back, in a middle finger to The Last Stand.
So basically everything is as Singer left it in X2, he's removed the films he wasn't involved in, and reclaimed his authorship.
Intrestingly, he even changes Magneto's costume from the comicy, colourful design of First Class, to something more resembling Ian McKellen's X2 costume.
This movie is literally Singer saying, "X-Men is mine!"
And all the better for it.
The X-Men trilogy: X-Men, X2, Days of Future Past, with First Class as a prelude.